In the past, there have been developed multi-fuel internal combustion engines which are able to be operated by the mixed combustion of a plurality of kinds of fuels. For example, in a first patent document, there is disclosed an internal combustion engine which uses a gas fuel as a main fuel, and a liquid fuel, as an ignition assist fuel. This first patent document describes that in cases where a required discharge voltage exceeds a limit voltage (design voltage) of a spark plug, the liquid fuel in addition to the gas fuel is supplied to the internal combustion engine as the ignition assist fuel.